Fuel-Cell
A fuel-cell reaction can be expressed as:2H2O+electrical energy⇄2H2+O2+potential electric energy.
A forward reaction of electrolysis consumes water (H2O) and outgases hydrogen (H2) and oxygen (O2). A back reaction of hydrogen and oxygen produces water and potential electric energy, i.e., the so called ‘fuel-cell’ reaction.
FIG. 1 shows a typical prior art reversible fuel-cell 100. A direct current power supply 110 supplies current through wires 120 to electrodes 130 inside an electrolysis cell 140. The current electrolyzes water 150 in the electrolysis cell 140 to produce a mixture of gaseous hydrogen and oxygen 160 which flow through an interconnecting tube 170 to a reagent reservoir 180. This pressurizes a liquid reagent 190, forcing the reagent to flow out from the reservoir via a tube 195.
In the prior art, a catalytic recombination of the hydrogen and oxygen 160 is considered and permitted. However, that recombination is slow and uncontrolled. Additionally, the uncontrolled recombination of hydrogen and oxygen in that way generates heat, which is not usefully reclaimable.
FIG. 2 shows the structure of a prior art reversible fuel-cell system 200. The fuel-cell 203 includes a positive electrode 205, a positive porous conductor 206, a pervious proton exchange membrane 207, a negative porous conductor 208, and a negative electrode 209. For clarity, these parts are shown as exploded. However, to achieve good electrical conductivity, the structures 205, 206, 207, 208, and 209 are immediately adjacent.
The membrane can be prepared by methods such as described in U.S. Pat. Nos. 3,041,317; 3,282,875 and 3,624,053. One membrane is a sulfonated tetrafluoroethylene polymer membrane produced by E. I. DuPont de Nemours and Company, and is sold under the trademark ‘Nafion’.
The polymer membrane 207 has the property that H+ ions can diffuse through the membrane. For use in fuel-cells, both sides of the membrane are coated with a carbon powder. The particles in the carbon powder are coated with a film of platinum. The platinum catalyzes the 2H2O⇄2H2+O2 reversible reaction. The carbon/platinum is in direct contact with a graphite-impregnated paper to provide a low-resistance electrical path, and the paper is highly porous to allow gas flow. The porous conducting layer is in turn contacted by perforated stainless-steel pressure plates, which are the actual mechanical electrode structures.
For electrolysis, the platinum catalyst on the carbon powder is decomposed due to energetic oxygen oxidizing the carbon powder. For this reason, cells that perform both electrolysis, that is, 2H2O+electrical energy →2H2+O2, and fuel-cell reactions, that is, 2H2+O2→2H2O+potential electrical energy, use a specialized catalyst on the oxygen side. Usually, this catalyst is platinum and rhenium, or platinum on rhenium oxide. These catalysts are also used in direct-conversion methyl alcohol fuel-cells.
Additionally, the fuel-cell 203 includes two separated gas-tight compartments, a hydrogen compartment 210 and an oxygen compartment 211. The two compartments are separated by the membrane 207′, also shown from the side. The membrane 207 keeps the hydrogen from mixing with the oxygen.
A direct current power supply 201 produces direct current. This current is switched by switch 202 to selectively supply current to a fuel-cell 203. Typically, the cell is initially filled with water. When the switch 202 is turned ‘on’, the current electrolyzes the water and produces hydrogen and oxygen.
After some quantity of hydrogen and oxygen are produced by the electrolysis, the switch 202 is turned ‘off’, and a load switch 204 is turned ‘on’ to supply current generated by the fuel-cell 203 to an energy recovering load 212. The presence of the load causes the hydrogen and oxygen to recombine. This current continues to be supplied to the load 212 as long as there is hydrogen and oxygen present in the cell.
When an external supply of hydrogen and oxygen is available, these gases can be supplied directly to the cell to generate electrical energy for the load 212 when load switch 204 is ‘on’. The load 212 can be an energy recovering load. That is, a device that returns potential electric energy to the system power supply 201. For a battery-powered device, this can be accomplished by using a switching power supply to recharge the supply 201. For an AC-line powered device, a line-synchronized DC-to-AC inverter can be used to return the potential electric energy.
Micropump
Various types of micropumps are known. Micropumps can be classified as mechanical or non-mechanical. Mechanical micropumps include electromechanically driven piston pumps and thermo-pneumatically driven peristaltic pumps. Non-mechanical micropumps include electro-hydrodynamic micropumps and magneto-hydrodynamic micropumps.
One electrochemical micropump is described by S. M. Mitrovski and R. G. Nuzzo, “An electrochemically driven poly(dimethylsiloxane) microfluidic actuator: oxygen sensing and programmable flows and pH gradients,” Lab on a Chip, Vol. 5, pp. 634-645, 2005. In this pump, oxygen and oxygen ions are used to ‘drag’ the fluid.
Other micropumps have been based on piezo effects in elastomer channels, Wei Gu, Xiaoyue Zhu, Nobuyuki Futai, Brenda S. Cho, and Shuichi Takayama describe one such system in “Computerized microfluidic cell culture using elastomeric channels and Braille displays,” Proceedings National Academy of Sciences of the United States of America, vol. 101(45), Nov. 9, 2004. In that pump, piezo actuators are used to form a peristaltic pump, alternately squeezing and relaxing a thin elastic tube.
A chip-based electrolysis-driven micropump is described by J. Liu et al., “Microfluidic Components for Chip-based Sample Preparation Systems,” The 50th ASMS Annual Conference on Mass Spectrometry and Allied Topics, Orlando, Fla., June, 2002. That micropump uses bubbles of hydrogen and oxygen to force reagents from the electrodes, which is electrolysis-based pumping. Clearly, that micropump is suitable only for reagents that are insensitive to hydrogen and oxygen. However, such reagents are rare, and that pump can only be used one time because there is no means to recombine the hydrogen and oxygen, nor any means to resupply the micropump with fluid.
It is desired to use electrolysis in a reversible fuel-cell as a mechanical energy generating device.